This invention relates to feedback loop control systems for lasers and more particularly to a feedback loop control system subject to a reference level control signal for controlling the periodic energization of a pulsed laser.
Pulsed lasers of all sorts, whether the laser medium is solid, liquid or gas, are subject to output fluctuations. There are short-term fluctuations which are small pulse to pulse variations in the peak output power and/or output pulse energy caused by small variations in stored energy, optical perturbations and others. There are also long-term fluctuations which are gradual changes in the average output energy (the average energy of each successive output pulse from the laser) caused by a number of factors including the following:
a. heating of the laser medium, PA0 b. heating and wear of electrical components, PA0 c. misalignment of the optical cavity, PA0 d. contamination of the optics, PA0 e. formation of absorbing or scattering species in the medium which affect the laser gain, and PA0 f. in a gas laser, consumption of the fuel gas.
Usually the long-term output of the laser declines with time, but it may also increase. For example, a gas laser may be filled with too much of one gas component and so the output of the laser would increase as that component is consumed and its concentration moves toward an optimum value (everything else being constant).
In some pulsed gas lasers, the gaseous medium flows through the optical cavity and this reduces the long-term degradation. This technique is typical with N.sub.2 and CO.sub.2 gas lasers. However, even for those gas lasers, the long-term gradual change in output pulse energy is not eliminated, it is only reduced.
The repetition rate of an "excimer" or rare gas halide laser is limited. In a static gas system, the same gas volume cannot be excited repeatedly and produce output radiation pulses of the same energy, unless the gas is allowed to return to the initial state between excitations which takes on the order of a second and so the pulse rate for successive equal pulses is limited to about one pulse per second. This limitation of an excimer laser can be overcome with a dynamic gas system where the gas flows through the excited area. Empirally, the gas volume must be exchanged two to four times between excitations to produce equal output pulses. Hence, it has been the practice with dynamic excimer lasers to flow the gas through the discharge area at rates of several liters per second. By doing this, higher repetition rates can be achieved.
The repetition rate of a gas laser, as explained above, is limited and empirical guidelines are available that relate repetition rate to gas flow rate through the active region of the laser. Theoretically, the gas molecules disassociate into their atoms following lasing and then are re-excited to form the parent molecules and so the process is completely reversible and a single gas fill should be usable for an unlimited number of discharges. In practice, however, there are molecular impurities in the gas that adversely affect the laser emission. Also, particles of material sputter off of the electrodes and scatter light and contaminate the optical windows. In addition, the fuel gas reacts and so is no longer available for excitation. All of these factors contribute to the relatively long-term degradation in the energy of the laser output pulses. A measure of this for a given laser system is the number of output pulses up to the point at which the energy of a pulse has fallen to fifty percent of the initial value. This is called the gas lifetime.
Clearly, for any gas laser the long-term degradation of the energy of the laser output pulses can be alleviated somewhat by flowing the gas through the active region from a relatively large gas reservoir and by removing impurities. However, for some gas lasers an unsealed or flowing or dynamic gas system is not practical and a sealed gas system is required, The reasons for this range from cost and size limitations to the excessively corrosive quality of the gas. And so for such lasers there is a need to provide a method and means of controlling the laser so that the energy of the output pulses remains substantially constant over the useful operating period (the lifetime of one gas fill).